The present invention relates to an apparatus for reducing or suppressing rocking motion such as rolling and pitching of a marine floating structure such as ship or barge.
Known apparatuses of this kind are an anti-rolling tank and a fin stabilizer. In an anti-rolling tank as schematically shown in FIG. 1, which is used to suppress rolling of a ship, a U-shaped water tank c with its upper ends being intercommunicated through an air pipe b is arranged in a main hull a on a deck above a center of gravity of the hull. It is designed such that in response to rolling of the hull a, water d in the tank c is caused to make passive resonance which lags in phase by 90.degree. to the rolling.
A fin stabilizer schematically shown in FIG. 2 comprises movable fins f which are driven by drives e and which are attached to a submerged bilge of a main hull a substantially at a mid-point thereof in a fore-and-aft direction. An angle of rolling, an angular velocity and an angular acceleration of the main hull a are detected by a sensor g and an angle of elevation of each fin f is actively varied in response to the detected values so that the rolling of the main hull a is suppressed by dynamic lift produced on the fins f according to a velocity of the ship.
An anti-rocking system using solid mass as shown in FIG. 3 has been also proposed in which a base h with rails i is mounted on a bottom, deck or the like of a main hull a and a weight k as solid mass rides through wheels j on the rails i. A lead-screw 1 threadably extends through the weight k and is supported at its opposite ends by bearings m. One end of the lead-screw 1 is connected through a coupling p to a reduction gear n and a motor o. When the main hull a rocks, the motor n is rotated in a clockwise or counterclockwise direction so that the weight k is displaced in a direction opposite to that of an external force acting on the hull a so as to reduce the rolling motion of the hull a.
The anti-rolling tank has the following problems:
(1) Since a required whole weight of the tank is generally 3 or 4% of a displacement of the hull in the case of a smaller vessel and 1 or 2% in the case of a larger vessel and since the anti-rolling mass is water, the system requires a large space over the upper deck, resulting in bad after-visibility in steering. PA1 (2) In a smaller ship, performance of the ship is adversely affected by the raised center of gravity. PA1 (3) An anti-rolling effect may be comparatively satisfactory in a ship in waves. But, when the waters are calm, free water increases so that an angle of list or heel of the ship is increased. PA1 (4) Once designed, the anti-rolling tank has a predetermined natural period so that when an actual rolling period of a ship is different from that of the anti-rolling tank designed, the anti-rolling effect is reduced. PA1 (5) Noise is produced in response to flow of the water and associated flow of the air through the intercommunicating pipe so that environmental conditions such as comfortability are adversely affected. PA1 (1) The anti-rolling effect cannot be ensured until the velocity of the ship becomes in excess of a certain level at which the fins produce dynamic lift. In other words, the anti-rolling effect is not expected when the ship moves at a low velocity or when the ship stops. PA1 (2) Rigging of the fin stabilizer to the hull is very complicated. PA1 (3) As compared with the anti-rolling tank, the fin stabilizer is very expensive (about five times as much as that of the anti-rolling tank). PA1 (4) There exists a fear that the noise produced by the fin stabilizer adversely affects functions of sonar equipment. PA1 (5) Since the fin stabilizer is attached to the hull, the velocity of the ship is slowed.
The fin stabilizer has the following problems.
In the case of the anti-rocking system utilizing solid mass, a drive includes a rotating body so that in order to linearly drive a driven body, an auxiliary system is required comprising gears and a screw (or a linkage). As a result, the construction becomes complicated and system failure frequently happens and consequently maintenance becomes cumbersome. In addition, the whole weight of the system becomes very heavy.
In view of the above, the present invention has for its object to provide an apparatus for reducing rocking motion of a marine floating structure, which is simple in construction and light in weight, by overcoming the above-mentioned problems encountered in the anti-rolling tank, the fin stabilizer and the anti-rocking system using solid mass.
In order to overcome the above-mentioned problems, an apparatus for reducing rocking motion of a marine floating structure in accordance with the present invention comprises a linear motor with stator means and movable means, said stator means being securely attached to the marine floating structure, said movable means serving as solid mass, a rocking-motion sensor for detecting rocking motion of said marine floating structure and a controller for phase-controlling a signal from said rocking-motion sensor to transmit a driving command to said linear motor.
When the rocking motion of the marine floating structure is detected by the rocking-motion sensor, a signal therefrom is phase-controlled by the controller and is outputted to the linear motor. In response to the received signal, the movable means is forced to move in a direction of reducing rocking motion against the marine floating structure in rocking motion so that the movable means of the linear motor acts as solid mass to reduce the rocking motion of the marine floating structure.
The present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.